This memoir attempts to give concise descriptions of all the fish species known to inhabit the waters of North America, north of the boundary between the United States and Mexico. The classification system adopted is essentially based on the views of Professors Gill and Cope rather than the fish groupings offered by the European systematists, with an effort to avoid unnatural associations in genera and families on one hand and subdivision beyond reasonable limits, on the other. Genera which have defied precise definition are rarely adopted.